This invention relates in general to the manufacture of color television picture tubes, and in particular to a method for dispensing frit material onto the seal land of a glass funnel constituting part of a color CRT bulb.
Conventional color CRT bulbs are fabricated in two parts, a funnel and a faceplate. The funnel and faceplate have seal lands which are joined together to form a hermetic seal. Conventionally, the funnel has a flat seal land (i.e., the seal land lies in a flat plane which is perpendicular to the axis of the funnel. The conventional faceplate is curved and has a flange with a flat seal land which matches the seal land of the funnel. The funnel and faceplate are joined by means of a frit material (a devitrifying solder glass cement - sometimes termed hereinafter simply a "frit"0 which is applied to the seal land of the funnel.
Apparatus used to dispense the frit material onto the funnel seal land typically holds the funnel in a vertical, neck-down position. A nozzle which dispenses the frit material moves around the glass funnel above the seal land and applied the frit material to the seal land. It is necessary that the seal land under a frit dispensing nozzle be in a horizontal plane so that the frit material will not run off until enough time has elapsed for the frit material to set up. Another type of apparatus is known wherein the nozzle is fixed and the funnel is rotated and translated in the plane of the seal land under the nozzle.
This invention is believed to be most useful when applied to a tube having a flangeless faceplate. Such a tube is disclosed in U.S. Pat. No. 3,894,260, issued to the assignee of this application. The tube has a flangeless, curved glass faceplate, a concave inner surface of which receives a phosphor screen. The funnel portion of this unique tube has a convex curved seal land (that is, the seal land defines a convex curved plane) which matches and mates with the curvature of the concave inner surface of the faceplate. Since the faceplate is flangeless, the sealing interface between the funnel and faceplate is curved rather than planar as in conventional tubes.
To adapt prior art frit-applying apparatus to apply frit to this type of glass funnel, the nozzle would have to be given an up and down motion as the funnel seal land is transversed. Apparatus capable of such movement would be very complex and precise (and therefore expensive) to insure that the frit material is applied evenly and in the center of the seal land. In such an adaptation of prior art apparatus, the seal land would not be in a horizontal plane when the tube is mounted in the vertical position. The frit material (as presently constituted) is not sufficiently viscous to prevent run-off from the seal land.